[numpy] Remove the duplicated {index} directive under Matrix Multiplication

The "Matrix Multiplication" section carried the same
`{index} single: NumPy; Matrix Multiplication` directive twice, back to
back, so the general index listed the entry twice. Remove the second.

The French edition already removed its copy (see
QuantEcon/lecture-python-programming.fr#24), which leaves its numpy.md
one directive short of the source; every French sync of numpy.md has
since failed the translation engine's structural-parity check. No prose
or code changes.

Closes #600

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/numpy.md

@@ -434,9 +434,6 @@ In particular, `A * B` is *not* the matrix product, it is an element-wise produc
 ```{index} single: NumPy; Matrix Multiplication
 ```
 
-```{index} single: NumPy; Matrix Multiplication
-```
-
 We use the `@` symbol for matrix multiplication, as follows:
 
 ```{code-cell} python3